Permanent magnets are developed for several important magnetic characteristics including high maximum energy content, high resistance to demagnetization, and high induction. Maximum energy content is important because permanent magnets are used primarily to produce a magnetic flux field which is a form of potential energy. The maximum energy content that is available for use outside the magnet body, commonly referred to as maximum energy product, BH.sub.max, is a well known indicator of the quality of a magnet. The higher the maximum energy product, the more energy available for use outside the magnet and thus, the better the magnet.
One measure of the resistance of the magnet to demagnetization is known as intrinsic coercivity, commonly referred to as "H.sub.ci ". In addition to having a high BH.sub.max, it is also important for a permanent magnet to have high resistance to demagnetization and thus, a high H.sub.ci. Magnets which have intrinsic coercivities above about 26 KOe range. although desirable, may not be practical, since it is difficult to magnetize them.
The advantages of samarium cobalt alloy magnets are now well-known. For example, such magnets are especially suitable for use in small electric motors and small appliances. However, one disadvantage to the use of magnets comprised of Sm.sub.2 Co.sub.17 alloys is that, while they provide adequate maximum energy products, they have intrinsic coercive forces which are too low for many applications. For example, in U.S. Pat. Nos. 4,210,471, issued Jul. 1, 1980, in the name of Yoneyama et al.; 4,213,803, issued Jul. 22, 1980, in the name of Yoneyama et al.; 4,284,440, issued Aug. 18, 1981, in the name of Tokunaga et al.; and 4,289,549, issued Sep. 15, 1981, in the name of Kasai, magnets are provided which have BH.sub.max values about 30 MGOe, however, the H.sub.ci values of these magnets are only about 6 to 8 KOe. Such magnets are not suitable for use in applications that require large electric DC motors, such as robots and major appliances.
Various attempts have been made previously to provide samarium cobalt permanent magnets having high intrinsic coercivities. While such improvements have resulted in the production of magnets with higher coercivity, this improvement has been offset by loss of other desirable properties, including maximum energy product, second quadrant loop squareness, and remanence. For example, U.S. Pat. No. 4,536,233, issued Aug. 20, 1985 in the name of Okonogi et al. discloses samarium cobalt permanent magnets having an H.sub.ci of about 15 KOe. However, the magnets were disclosed to have a BH.sub.max of only about 16 MGOe. This is significantly less than the BH.sub.max of about 30 MGOe as disclosed by others. Similarly, U.S. Pat. No. 4,565,587, issued Jan. 21, 1986 in the name of Narasimhan discloses a Sm.sub.2 Co.sub.17 permanent magnet having a maximum energy product of about 24 MGOe with an intrinsic coercivity of about 18 KOe. U.S. Pat. No. 4,497,672, issued Feb. 5, 1985 in the name of Tawara et al., discloses a method for producing samarium cobalt permanent magnets having maximum energy products of about 22 MGOe and intrinsic coercivities of about 23.4 KOe. Thus, improved intrinsic coercivity in a permanent magnet has only been obtainable heretofore at the expense of a decrease in maximum energy product.
In addition to maximum energy product and intrinsic coercivity, there is another important figure of merit for determining the quality of permanent magnets. This parameter, known as second quadrant loop squareness ("H.sub.k "), is a measure of how square the demagnetization curve is. By definition, it is the H value (H is the demagnetizing force) measured at a magnetization 10 percent down from the residual induction B.sub.r. In practical terms, H.sub.k is indicative of how much energy can be stored in the field. High H.sub.k and H.sub.ci values are desirable because they reflect an increase in the stability of the magnet. In addition, these properties are important since they affect the required geometry of the magnet. Thus, they are determinative, in part, of whether a magnet will perform well in certain applications.
Various attempts to produce samarium cobalt magnets that have, at once, high maximum energy products, high intrinsic coercivities and high second quadrant loop squareness, have not been successful. For example, in U.S. Pat. No. 4,746,378, issued May 24, 1988 in the name of Wysiekierski et al., there is disclosed a process for producing an alloy which can form magnets having maximum energy products of 30 MGOe. However, the intrinsic coercivities are only about 14-16 KOe, and the second quadrant loop squareness is only about 9.0 KOe for these magnets.
In Paper No. 18PO227 of the 10th International Workshop on Rare-Earth Magnets and Their Applications, Kyoto, Japan, May 16-18, 1989, Edeling and Herget described a process for producing magnets having improved intrinsic coercivities and improved squareness. However, these magnets are disclosed to have been made from calciothermic, co-reduced alloys that contain non-magnetizable oxide and carbide impurities. The presence of these impurities in the alloy starting materials results in a decrease in the flux related properties BH.sub.max and B.sub.r and thus, the quality of the magnets produced. Edeling and Herget indicate that the results they obtained could not be duplicated using magnets prepared from melted 2-17 alloys.
Despite many efforts directed to producing improved magnets made from Sm.sub.2 Co.sub.17 alloys, the magnets produced by prior processes do not possess, at once, good flux properties, including high maximum energy products, as well as high intrinsic coercivities and high second quadrant loop squareness. Accordingly, there is a need for permanent magnets which exhibit unique combinations of these desirable magnetic properties.